1. Field of the Invention
The present invention pertains to a method for determining the gas fraction in a multiphase (liquid and gas) fluid stream by measuring induced vibrations of the fluid stream to determine a resonant vibration frequency.
2. Background
Various systems and methods have been developed to measure the volumetric and mass fraction of the components of a multiphase (gas and liquid) fluid flowstream. In the production of oil and gas, for example, it is particularly desirable to be able to measure the gas fraction of the production flowstream emanating from one or more wells without requiring separation of the gas from the liquid at the wellhead, for example. Various devices and systems have been developed for measuring the volumetric fraction of gas in the total flowstream, including systems which utilize gamma ray type densitometers, coriolis type flow meters, differential pressure measurement systems and dielectric constant measurement devices. Systems which utilize gamma ray type densitometers suffer from low sensitivity and the burdens of managing nuclear devices. Coriolis type meters have a pressure limit of about 1500 psig and a gas fraction limit in the flowstream of about 10% to 20%. Differential pressure measurement systems are relatively complicated and dielectric constant measurement devices sometimes suffer from poor accuracy at high gas fraction conditions in the flowstream and they are sensitive to changes in the components of the flowstream such as, wherein the liquid component is a mixture of oil and water.
Miroslav M. Kolpak U.S. Pat. Nos. 5,090,253 and 5,259,250 assigned to the assignee of the present invention disclose and claim multiphase fluid flow measurement systems wherein the density and gas fraction of the flowstream may be determined by vibrating a tube containing the flowstream over a range of frequencies and measuring the phase angle and amplitude of the fluctuating fluid pressures of the flowstream compared with the acceleration of the tube to determine the "sloshing" natural frequency of the fluid mixture. However, the power requirement for vibrating the tube or tubes may exceed that which would be available at remote measurement locations, such as wellheads located in Arctic oil fields of the North Slope of Alaska, for example. Accordingly, there continues to be a need to develop a multiphase fluid flow measurement system and method which is particularly attractive for measuring the gas fraction in the total fluid flowstream on a volumetric basis, in particular. It is to this end that the present invention has been developed.